1. Field
Exemplary embodiments of the present invention relate to a termination circuit, which terminates an interface node for data output or impedance matching.
2. Description of the Related Art
Semiconductor devices including integrated circuit (IC) chips such as CPU, memory, and gate array are used in different electronic products such as personal computers, servers or workstations. Semiconductor devices often include a receiving circuit for receiving various signals transmitted from outside through an input pad and an output circuit for providing an internal signal to the outside through an output pad.
Meanwhile, with the increase in operation speed of electronic products, the voltage swing of a signal exchanged between semiconductor devices has gradually decreased to minimize a delay time in signal transmission. However, with the increase of the signal swing, external noise effect increases, and signal reflection caused by impedance mismatching at an interface terminal becomes more pronounced. The impedance mismatching is caused by external noise, variation in power supply voltage, change in operation temperature, change in fabrication process or the like. When the impedance matching occurs, it is difficult to transmit data at a high rate, and data outputted from a data output terminal of a semiconductor device may be distorted. Therefore, when a semiconductor device in a receiver side receives the distorted output signal through an input terminal thereof, undesirable features such as setup/hold fails or input level decision errors may frequently occur.
In particular, a memory device of which the operation speed is to be increased employs an impedance matching circuit called an on-die termination circuit, which is located around an input pad inside an IC chip, in order to address the above-described features. In a typical on-die termination scheme, source termination is performed by an output circuit in a transmitter side, and parallel termination is performed by a termination circuit connected in parallel to a receiving circuit connected to the input pad in a receiver side.
FIG. 1 is a configuration diagram of a conventional termination circuit. The termination circuit is used as an output driver when data (signal) is outputted and performs impedance matching of an interface pad when data (signal) is inputted.
Referring to FIG. 1, the termination circuit includes a pull-up termination unit 110, a pull-down termination unit 120, and a control unit 130.
The pull-up termination unit 110 is enabled in response to a pull-up signal PU_EN, and pull-up terminates an interface pad INTERFACE through which signals or data are inputted/outputted. A pull-up code PCODE<0:N> inputted to the pull-up termination unit 110 controls an impedance value of the pull-up termination unit 110. That is, whether the pull-up termination unit 110 is enabled or not is decided by the pull-up signal PU_EN, and the impedance value of the enabled pull-up termination unit 110 is controlled by the pull-up code PCODE<0:N>.
The pull-down termination unit 120 is enabled in response to a pull-down signal PD_EN, and pull-down terminates the interface pad INTERFACE. A pull-down code NCODE<0:N> inputted to the pull-down termination unit 120 controls an impedance value of the pull-down termination unit 120. That is, whether the pull-down termination unit 120 is enabled or not is decided by the pull-down signal PD_EN, and the impedance value of the enabled pull-down termination unit 120 is controlled by the pull-down code NCODE<0:N>.
The control unit 130 is configured to control whether or not to activate the pull-up signal PU_EN and a pull-down signal PD_EN. The operation of the control unit 130 is divided into a case where data is outputted to the interface pad INTERFACE and a case where data is inputted to the interface pad INTERFACE. Based on both cases, the operation of the control unit 130 will be described as follows.
(1) Operation of Control Unit 130 when Data is Outputted
When an output data value DATA is high, the control unit 130 activates the pull-up signal PU_EN. When the pull-up signal PU_EN is activated, the pull-up termination unit 110 is enabled. As a result, high-level data may be outputted to the interface pad INTERFACE. When the output data value DATA is low, the control unit 130 activates the pull-down signal PD_EN. When the pull-down signal PD_EN is activated, the pull-down termination unit 120 is enabled. As a result, low-level data may be outputted to the interface pad INTERFACE.
(2) Operation of Control Unit 130 when Data is Inputted
When data is inputted to the interface pad INTERFACE, a termination signal iODT is activated. The termination signal iODT is a signal to terminate the interface pad INTERFACE when data is inputted to the interface pad INTERFACE. When the termination signal iODT is activated, the control unit 130 activates both of the pull-up signal PU_EN and the pull-down signal PD_EN. Therefore, both of the pull-up termination unit 110 and the pull-down termination unit 120 are enabled, and the interface pad INTERFACE may be impedance-matched.
As described with reference to FIG. 1, the termination circuit outputs data to the interface pad INTERFACE using the same termination units 110 and 120, and matches the impedance of the interface pad INTERFACE when data is inputted. Accordingly, it is desirable to obtain an index indicating how much the impedance values of the pull-up termination unit 110 and the pull-down termination unit 120 are mismatched during the impedance matching operation of the termination circuit. Therefore, when the impedance values of the pull-up termination unit 110 and the pull-down termination unit 120 are mismatched, the impedance values are to be corrected. When the impedance values of the pull-up termination unit 110 and the pull-down termination unit 120 are corrected, impedance values during data output may also be changed. That is, according to the configuration of the conventional termination circuit, it is difficult to separately control the impedance values of the termination units 110 and 120 during data output and the impedance values of the termination units 110 and 120 during data input. Therefore, it is difficult to control the termination circuit to have optimal impedance values.